Over 40 million Americans (14 million of which are men) of all ages are afflicted with low bone density and its associated increased risk of fractures. While aging is a major cause of osteoporosis, diseases (such as type I diabetes, cystic fibrosis, asthma, inflammatory bowel disease, and arthritis), inactivity, poor dietary choices, and certain medications can cause bone loss at any age. Recent studies demonstrate that bone communications with the intestine. Potential mechanisms include signaling to bone through gastrointestinal (GI) hormones and calcium absorption. Recently, we found another interaction: GI inflammation leads to increased bone inflammation, which is associated with suppressed bone formation and bone loss in mice. This suggests that therapies which improve overall intestinal health have the potential to benefit bone health. Identifying natural products that could help individuals attain their maximum bone mass and suppress bone loss would be of great preventative and therapeutic value. Probiotics, microorganisms that provide a health benefit to the host when ingested, are used to treat a wide variety of ailments ranging from diarrhea to allergies. These microorganisms may boost the immune system, reduce colonization of harmful bacteria in the intestine, destroy certain bacterial toxins, decrease intestinal inflammation and help with digestion and mineral absorption. To address the effects probiotics on bone, we administered an anti- inflammatory probiotic strain, Lactobacillus reuteri ATCC PTA 6475, to adult mice and measured their bone densities. Fascinatingly, we found that L. reuteri treatment decreased TNF levels in the ileum and increased bone volume by greater than 45% in healthy male but not female mice. Based on our data we hypothesize that ingestion of L. reuteri increases bone density in a gender dependent manner through suppression of intestinal inflammation and upregulation of bone formation. We further hypothesize that sex hormones and changes in other intestinal signals (microbiota composition, hormones, and calcium status) contribute to the bone response. We propose to test our hypotheses by 1) Identifying the influence of gender on bone responses to L. reuteri treatment and 2) Identifying characteristics of probiotic L. reuteri and its effects on the gastrointestinal tract which are critical for enhancing bone mass. Few studies have examined probiotic effects on bone, and none have included an extensive analysis of bone, serum and GI parameters. Our studies will fill this gap in knowledge by identifying basic mechanisms relating the GI tract to bone health and will test the influences of gender in this linkage. Given the increasing use of probiotics in the United States and worldwide, our findings warrant further investigation into the effects of probiotic use on bone health in males and females. PUBLIC HEALTH RELEVANCE: Finding effective novel treatments for bone loss is a priority, since it is estimated that by 2020 more than 61 million men and women will have osteoporosis and its associated increase in fracture risk and potential negative effects on metabolism and insulin secretion. We have identified a novel way of increasing bone mass by use of a probiotic bacterium that attenuates intestinal inflammation. If proven effective, it could lead to a significant improvement in the health and well being of people with decreased bone density.